The present invention relates to the design of an antenna system including both a band-pass antenna and a band-reject antenna. More particularly, this invention relates to two independently steered antennas which are closely spaced, one antenna being band-pass and the other antenna being band-reject.
The design of microwave antennas for use in congested environments such as in the top of the stabilizer of an aircraft, places stringent limitations on the location of the antennas. Furthermore, it is often necessary to pack several antennas into a small area and consequently the antennas tend to be close together. This close proximity of the antennas can cause interference and unwanted reflections distorting the patterns of the antennas.
In order to compensate for the unwanted effects of antennas in close proximity, dichroic or frequency sensitive surface (FSS) reflectors are commonly selected to form part of an antenna which is in close proximity to one or more other antennas. In such reflective antenna structures it is possible to design antenna reflectors, that will pass signals of one frequency band and reflect signals of other frequencies. An FSS is designed such that it selectively transmits some wavelengths of radiation and reflects others. The FSS can thus enable the separation of two bands, such as the Ka and Ku bands or L and Ku bands.
The reflectors are normally made up of a large array of resonant elements, known as resonators. These resonators may be dipoles, of various configurations. The dipoles reflect certain frequencies while the FSS also transmits the other frequencies. Based on the relative size of the dipoles, in relation to the wavelength, the reflection and transmission of frequencies is altered. Proper sizing of the dipoles will then determine the frequencies reflected while other frequencies are transmitted.
The dipole resonators are grouped into a grid formation on an antenna reflector to form a frequency-sensitive surface. The spacing between the resonator elements is an important design constraint in differentiating the reflected and transmitted bands and can strongly influence the bandwidth of she rejection band.
In the prior art, U.S. Pat. No. 3,842,421, issued to Rootsey, discloses an antenna having a reflective surface with an array of apertures. The apertures have a resonant character independent of polarization. The reflective surface itself is thereby transmissive at the resonant frequency and reflective at frequencies sufficiently removed from resonance. Rootsey also discusses another alternative, where the metal-dielectric patterns are reversed. For example, an array of cross holes may be cut into a metal plate and then mounted onto a dielectric substrate. According to Rootsey, the cross holes are resonant at certain frequencies such that the antenna is transparent at those frequencies. The Rootsey patent defines resonant elements comprising apertures in a conductive surface and producing frequency selective energy transmission. The Rootsey patent may be applicable to any range of frequencies, more particularly it will function in the L-band, Ku-band and Ka-band. While Rootsey discloses an antenna element consisting of resonant holes, a dual antenna system comprising a transmissive surface antenna and a reflective surface antenna is not disclosed.
In U.S. Pat. No. 5,982,339, Lalezari discloses an antenna system consisting of a primary antenna and a secondary antenna. Lalezari teaches a primary antenna having a frequency selective surface portion for transmitting a first frequency and being predominantly transmissive to a second frequency. The second antenna element, taught by Lalezari, transmits a second frequency while the FSS of the second antenna element is transparent to the first frequency. However, Lalezari requires that the resonant element of the primary antenna form a continuous path of metalized segments and the configuration does not correspond to a bandpass/band-reject pair of antennas as proposed here. Instead, the structure has two band-pass surfaces tuned to different frequencies. Non-interconnected elements will have a band pass or band-reject at harmonics of a single frequency. Interconnected elements will result in multiple fundamental frequencies with their associated harmonics. The non-interconnected elements thus provide improved discrimination. Lalezari does not teach the use of non-interconnected resonant elements as an FSS which may be tuned according to the frequency required.
In view of the above, the present invention provides a dual antenna system utilizing frequency selective surfaces having non-interconnected resonator elements and resonant openings, respectively. The present invention further provides a first antenna element which reflects a first frequency while being transparent to a second frequency. The second antenna element, provides the reverse, and reflects the second frequency.
The present invention provides a dual antenna system where a first antenna element has a metallic surface with openings that are resonant at frequencies other than the operating frequency of a second antenna element. The openings are sized such that the metallic components are relatively transparent at and near resonant frequencies of those openings. According to the present invention, the resonant frequencies of the openings may be the transmitting or receiving frequencies of the second antenna element, or of nearby antennas.
In a broad aspect, the present invention seeks to provide a complementary dual antenna system comprising:
a first antenna comprising at least one first element on a structure for transmitting and receiving signals within a first frequency band;
a second antenna comprising at least one second element spaced apart from said at least one first element on the structure for transmitting and receiving signals within a second frequency band;
said first antenna having a first frequency selective surface, said first frequency selective surface having a resonant grid pattern formed by the said at least one first element, wherein each of said at least one first element is tuned such that the first element is resonant and transparent at a frequency within said first frequency band and reflective at a frequency within said second fluency band; and
said second antenna having a second frequency selective surface, and said second frequency selective surface formed by said at least one second element, said at least one second element each comprising a resonant opening tuned such that the said resonant opening is resonant and transparent at a frequency within said second frequency band and reflective at a frequency within said first frequency band.
Preferably, each of the resonant opening has a length which is optimized to reduce the radar cross-section of the second antenna at a frequency within the first frequency band.
Preferably, the separation between each of the resonant openings is optimized to provide a maximum bandwidth to the second frequency band.
Preferably, the second antenna element operates in an L-band frequency range.
Preferably, the resonant elements are rings, cross dipole square hoops, Jerusalem crosses or tripoles.
Preferably, the resonant openings are complementary of the resonant elements.